Much activity is presently being directed into the design and deployment of "point-to-multipoint" broadband access networks, wherein downstream signals are broadcast from a single head-end facility to multiple end user stations (i.e., via "point-to-multipoint" transmission), and upstream signals are transmitted from each respective end users to the head end facility (i.e., via "point-to-point" transmission), respectively. It presently anticipated that point-to-multipoint broadband access networks will be employed to support a variety of independent communication services, such as, e.g., traditional two-way telecommunications, broadcast video (i.e., CATV) services and a full range of digital baseband services.
Given the wide variety of potential communication services to be supported over point-to-multipoint network broadband access networks, it is desirable to provide efficient digital data transmission protocols for supporting both the downstream and upstream communication paths. Notably, such networks are well suited to support asynchronous transfer mode ("ATM") based data transmission, whereby data packets or "cells" are periodically assembled and transmitted from a sending node, and received and disassembled at a receiving node.
In particular, ATM transmission enables the transmission of multiple services over a single communication path, whereby individual service bandwidth utilization may be optimized as a function of the statistical activity of each individual service--i.e., wherein the services are "bursty" in nature, so that bandwidth is efficiently shared.
For example, bursty data traffic may include local area network ("LAN") traffic, which is traditionally limited to private, or closed-loop networks, but may be more frequently carried over shared public access (e.g., telecommunication) networks for greater efficiency in connecting multiple LAN locations. Further, with the explosion of recent interest in services associated with the "Internet", demand for low cost, high speed two-way digital data transport is at an all time high.
By way of specific example, network architectures and data communication protocols for supporting both downstream and upstream transport of ATM-based digital data over a point-to-multipoint network comprising a headend facility connected to multiple downstream network terminals ("NTs") via a shared coaxial distribution network are disclosed and described in U.S. patent application Ser. No. 08/772,088, filed Dec. 19, 1996, entitled "Network Architecture for Broadband Data Communication Over a Shared Medium" (hereinafter referred to as "the '088 application"), which is fully incorporated herein by reference.
As described therein, downstream data is transmitted point-to-multipoint over the coaxial distribution network in continuous serial data frames over a common RF carrier frequency, whereby all downstream data frames are received by all NTs associated with the respective downstream RF carrier channel. The NTs evaluate each downstream data frame based on, e.g., destination or broadcast address fields, to determine whether it is an intended recipient of data contained therein. Upstream data is transported point-to-point in respective time-divided data slots from individual NTs to the headend in over a shared upstream RF carrier channel, whereby the upstream slots form successive upstream data frames at the headend.
In order to control upstream transmission, the headend allocates upstream "bandwidth" (i.e., upstream transmission slots) to respective NTs based on a selected set of operating criteria and service type priorities. To this end, each downstream data frame includes an upstream bandwidth "permit" transmitted as part of a media access control ("MAC") level protocol that, depending on the type of permit issued in a given downstream frame, allows for one or more respective NTs to transmit a specified response in a corresponding upstream slot.
Generally, permits are issued by the headend on either a "reserved" basis, i.e., wherein an upstream response is allowed only by a specified NT, or a "contention" basis, i.e., wherein one or more NTs may attempt a response in a single upstream slot. As will be apparent to those skilled in the art, there are three possible outcomes in the later case: (1) only a single NT will respond to the contention permit and will, thus, be successful; (2) two or more NTs will attempt to respond to the contention permit, thereby causing a collision in the designated upstream slot, with neither NT's response being successful; or (3) no NT will attempt a response.
In an effort to allow for a greater number of upstream bandwidth requests from the various NTs to reach the headend, the headend will periodically issue a "multi-slot" contention permit, wherein a single downstream permit allows a response (albeit substantially shortened) in any one of several successive "mini" upstream response slots. Of course, the possible outcomes are the same for any one of the slot positions as they are when only a single slot is available, i.e.: (1) only a single NT will respond in a given slot position and will be successful; (2) two or more NTs will attempt to respond in the same slot position, thereby causing a collision; or (3) no NT will attempt a response in a particular slot position.
The respective unsuccessful NTs will normally re-attempt an upstream response to the next multi-slot contention permit. In accordance with known practices, if an NT is not successful with its initial response in the selected slot position, it will keep with that slot position for attempting a response to each successive multi-slot permit, until such time as it successfully transmits a response. Thus, in order to minimize the chance for further collisions by NTs vying for the same slot position, a "re-transmission" protocol is preferably employed. For example, as taught in the '088 application, known contention resolution random access algorithms (RAAs) are employed, such as the Ternary Tree Binary Feedback (TTBF) algorithm, whereby respective NTs are assigned differing priority numbers for re-attempting transmission after a collision.
However, the aforedescribed algorithm is problematic, in that it does not successfully address situations where a majority of NTs happen to randomly select, and repeatedly vie for, only a few of the upstream slot positions, thus greatly decreasing the transmission success rate in those particular slot positions, while possibly leaving the remaining slot positions under-utilized.